Combustion air system for recovery boilers, burning spent liquors from pulping processes

ABSTRACT

An arrangement is disclosed for supplying an air jet form to the furnace of a recovery boiler, where the furnace has a front wall, a rear wall and side walls. Black liquor spraying devices are disposed on the furnace walls at one or several levels of the furnace. A plurality of air ports are located at several horizontal levels for introducing air into the furnace from an air supply. In the arrangement for the secondary air flows, two horizontal air levels at different elevations are arranged above the lower primary levels and below the black liquor sprayer. Secondary air is supplied from two opposite walls and the air ports from each of the two levels are located so that the air jets are introduced in an interlaced pattern having an even number of jets on one opposite wall and an uneven number of jets on the other opposite wall. The air jets of said at least two air levels are located substantially one above each other in substantially vertical rows.

RELATED APPLICATION

[0001] Priority is claimed to U.S. Provisional Patent Application SerialNo. 60/281,791, filed on Apr. 6, 2001, and which is incorporated byreference herein.

FIELD OF INVENTION

[0002] The present invention relates to an arrangement for supplying airin an air jet form to the furnace of a recovery boiler. The furnace hasa front wall, a rear wall and side walls. Black liquor spraying devicesare disposed on these walls at one or several levels. A plurality of airports are located on several horizontal levels on said walls forintroducing air into the furnace from an air supply. Specifically, theinvention relates to an arrangement for organizing the secondary airflows below the black liquor spraying devices.

BACKGROUND OF INVENTION

[0003] An optimal supply of combustion air in the lower part of thefurnace of a black liquor recovery boiler plays a considerable role inthe control of a combustion process in the boiler.

[0004] Since the chemical reactions in the kraft recovery boiler arevery rapid, the speed of the process becomes substantially dependent onthe mixing of combustion air and black liquor. This mixing stepdetermines the burning rate and also has an effect on the processefficiency. Air and black liquor are typically introduced into theboiler through individual ports, and it is particularly important that arapid mixing in the boiler is effected by the air supply withoutgenerating large differences in the upward flow profile. The highvelocity “lift” in the center of the furnace is especially harmful as itresults in carry-over of the sprayed liquor droplets. The burningsymmetry must be controlled throughout the whole cross-sectional area ofthe boiler and the air supply must be adjusted when required.

[0005] Black liquor is generally introduced in the form of considerablylarge droplets into a kraft recovery boiler so as to facilitate thedownward flow of the droplets, and to prevent them from flowing,unreacted (as fine fume), upwards together with the upward flowing gasesto the upper part of the boiler. The large droplet size, which resultsin the droplets being spaced further from each other than in a fineblack liquor spray, means that proper mixing is even more important in arecovery boiler. Pyrolysis of black liquor solids produces char as wellas combustible gases. The char falls down to the bottom of the furnaceand forms a char bed which must be burned.

[0006] A stoichiometric amount of air, relative to the amount of blackliquor, is introduced into the recovery boiler and additionally, asurplus amount of air is supplied to ensure complete combustion. Toomuch excessive air, however, causes a loss in efficiency of the boilerand an increase in costs. Air is usually introduced into the boiler onthree different levels: primary air at the lower part of the furnace,secondary air above the primary air level but below the liquor nozzles,and tertiary air above the liquor nozzles to ensure complete combustion.Air is usually introduced through several air ports located on all fourfurnace walls, or only on two opposing walls of the furnace.

[0007] Primary air is typically 20-35% of the total air supplied intothe furnace, depending on liquor and dry solids content of the liquor.The task of the primary air is to keep the char bed from rising into airports of the furnace. Secondary air is typically 35-60% of total air,and tertiary air, which may be distributed into several levels invertical direction, is typically 10-40% of the total air. More thanthree air levels for introducing air into the furnace may be arranged inthe boiler.

[0008] Mixing of black liquor and air is difficult because of the upflowof gas which is formed in the center part of the boiler, through whichit is difficult for the weak secondary air flow to penetrate. Morespecifically, the primary air flows, supplied from the sides in thebottom part of the boiler, collide with each other in the center part ofthe boiler and form, with secondary air flow pattern, in the center partof the boiler, a gas flow flowing very rapidly upwards, catching fluegases and other incompletely burnt gaseous or dusty material from thelower part of the furnace. This gas flow, also called a “droplet lift”,also catches black liquor particles flowing counter-currently downwardsand carries them to the upper part of the boiler, where they stick tothe heat surfaces of the boiler, thus causing fouling and clogging. Inthe center part of the boiler, the speed of the upwards flowing gas maybecome as much as four times as great as the average speed of the gasesas a result of incomplete or weak mixing. Thus, a zone of rapid flow isformed in the center part of the boiler, and this renders mixing of fluegases from the side of the flow very difficult to achieve.

[0009] The “droplet lift” mentioned above, results in such a situationwhere the tertiary air(s) has (have) to burn not only the unburned gasesfrom combustion (CO, H₂S, NH₃, etc.), but the unburned char from thedroplets as well. As the combustion rate for char is much slower thanfor the unburned gases, increased excess oxygen has to be used to ensurecomplete combustion. Then the flue gas leaving the furnace containshigher amounts of residual CO and H₂S, and the utilization of thefurnace is less effective than would be possible.

[0010] Current secondary air arrangements are also characterized by atleast one secondary air level where secondary air ports are placed closeto another in horizontal direction. This leads to mixing patterns wherefurnace gases are circulated in vertical direction, with the abovementioned “lift”, i.e. they flow towards the walls and then turn up (ordown) and follow the main flue gas direction.

[0011] Another variation of the secondary air design is to use partialinterlaced jets (e.g. U.S. Pat. Nos. 5,121,700, 5,305,698), whereby alarge jet opposes a small jet. The large and small jets are alternatedbetween the two opposite walls used.

[0012] U.S. Pat. No. 5,724,895 discloses an arrangement for feedingcombustion air. In this system, a more favorable flow pattern infurnaces can be achieved by replacing vertical mixing by horizontalmixing, whereby a strong central flow channel, upward “lift”, can beprevented. This horizontal mixing is applied for the whole furnace. Thehorizontal mixing is improved by disposing additional air inlet portse.g., at more than six different elevations in a pattern of verticalspaced-apart rows above the lowest air levels.

[0013] In the method of U.S. Pat. No. 5,454,908 a portion of combustionair is introduced into a recovery boiler at a distance above the blackliquor inlet so as to provide a reducing atmosphere with a residencetime of at least three seconds between the black liquor inlet and theintroduction of said portion of combustion air. A drawback of thedescribed arrangement is a high vertical combustion area, reaching inextreme cases the bullnose of the furnace. As this combustion area has areducing atmosphere, at least locally, more expensive materials have tobe used in the furnace to a higher position than would be needed ifcombustion took place lower in the furnace. Other disadvantages of theair systems, where combustion takes place high up in the furnace includehigh furnace outlet temperature resulting in large convective heattransfer surfaces later in the boiler, lower temperature in the lowerfurnace, and more expensive layout. The lower temperature in the lowerfurnace does not allow as high sulfidity without SO₂ emissions as acombustion system having a higher lower furnace temperature does.

SUMMARY OF INVENTION

[0014] The present invention provides an improved air supply system ofcombustion air to a recovery boiler. Particularly, a secondarycombustion air supply is provided in which either local and/or centralupward gas flows having a high velocity compared to an average upwardgas velocity are efficiently avoided. Another feature of the inventionis to enable a constant penetration of combustion air into the boiler atdifferent loading levels. A further feature of the invention is toproduce a better mixing of black liquor and combustion air in thefurnace. The improved air supply arrangement of this invention is alsodesigned to reduce the amount of harmful emissions from the boilerfurnace.

[0015] The present invention may be embodied in a recovery boiler havinga furnace that comprises:

[0016] two horizontal air levels at different elevations are arrangedabove the lowest air level or levels and below the black liquor sprayinglevel or levels,

[0017] air is supplied from two opposite walls on said two levels andthe air ports on each level are located so that the air jets areintroduced in an interlaced pattern having an even number of jets on oneopposite wall and an uneven number of jets on the other opposite wall,and

[0018] the air jets of said two air levels are located in substantiallyvertical rows.

[0019] According to an embodiment of the invention, secondary air on twoair levels is introduced only from the two opposite walls, preferablyfrom the front and rear walls. Substantially no air is supplied from thetwo remaining walls, i.e., the side walls. In the interlaced pattern, anair flow coming from an air port located on a wall having an even numberof air jets is directed in between two adjacent air ports of theopposite wall having an uneven number of air jets. Correspondingly, theair jets coming from the wall having an uneven number of air jets aredirected substantially directly in a horizontal plane towards theopposite wall. The air jets coming from the opposite walls by-pass eachother without actually colliding with each other.

[0020] Thus, on the two secondary levels, the lateral arrangement of thejets on one level sideways is symmetrical. On the wall having an unevennumber of air jets, e.g. three, the middle air jet is locatedsubstantially on the center line of the wall, and the other jets arelocated within an equal distance on both sides of the middle jet. On theopposite wall having an even number of jets, two in this example, thejets are located laterally midway between the jets on the opposite wall.Thus, the jet arrangement is symmetrical in relation to the verticalplane parallel to the remaining walls (i.e. the walls having nosecondary air jets) and passing through the center lines of the wallshaving the secondary air jets.

[0021] The present invention employs the following principles in orderto avoid strong vertical gas flows, but still to obtain effective mixingin the furnace between combustion air and unburned/burning liquordroplets:

[0022] strong secondary air jets (strong air jets below black liquorspraying devices).

[0023] arrange these jets so that they do not collide against eachother, which easily generates strong upflow jets and unwanted upflowprofile for the gases in the furnaces. Instead, strong shearing flowsshould be generated to obtain good mixing.

[0024] minimize suction of gases in vertical direction into these jetsabove the liquor spraying devices as this increases gas flow up.

[0025] minimize suction of liquor droplets from liquor sprays intotertiary air jets.

[0026] cover the tertiary air stage(s) with several jets, which coverthe furnace cross section evenly and well in order to prevent theformation of vertical jets that may punch the final combustion areawhere the final combustion of the unburned gases cannot take place.Also, here the jets should not collide against each other but generatestrong shearing flows and good mixing.

[0027] According to a preferred embodiment of the invention, there is adistance, V, in vertical direction between the horizontal air levels,when measured from the lateral center lines of the air ports of the airlevels. This distance, V, fulfills the following formula: V/L≦0.5, whereL is the distance between two adjacent air ports on the same air level,when measured from the longitudinal center lines of the adjacent airports. Preferably, V/L is 0.25-0.5. Typically, the vertical distance, V,is 1-2 meters.

[0028] Preferably the air ports located one above the other arepositioned in a vertical row so that they are located on the samestraight vertical line. The invention covers also an embodiment in whichthe air ports laterally deviate so that there a transverse distance, D,between the air ports above each other. The transverse distance is adistance between the longitudinal center lines of the ports one abovethe other. D is less than 1.5×H or less than 1.5×W depending on whichnumber is greater. H is the height of the highest air port and W is thewidth of the widest air port.

[0029] According to an embodiment of the invention there is only one airlevel below the two secondary air levels. According to anotherembodiment, the number of the lowest air levels below the two secondaryair levels is two. The air jets of the air level which is located higherin vertical direction below the two secondary air levels are arranged inan interlaced pattern on two opposite walls, preferably on the front andrear walls, so that the number of air jets is greater by one than thenumber of air jets of the two secondary air levels on the same wall. Forexample, if the secondary air level has one air jet on the front walland two jets on the rear wall, the above-mentioned lower air level hastwo air jets on the front wall and three jets on the rear wall. However,the air velocity is lower on this lower air level. On this air levelwhich, thus, is located above the lowest air level and below the twosecondary air levels, and which can be called a low-secondary orhigh-primary air level, the air jets are arranged also on the remainingopposite walls, i.e., preferably on the side walls. The air jets on theside walls are smaller than the air jets on the front and rear walls.

[0030] An embodiment of the invention is directed also to an arrangementfor supplying secondary air in an air jet form to the furnace of arecovery boiler, said furnace having a front wall, a rear wall and sidewalls, black liquor spraying devices disposed on said walls on a leveland a plurality of air ports located on several horizontal levels onsaid walls for introducing air into the furnace from an air supply, saidarrangement comprising: one horizontal air level is arranged above thelowest air level or levels and below the black liquor spraying level orlevels, and air is supplied from two opposite walls on said level andthe air ports are located so that the air jets are introduced in aninterlaced pattern having an even number of jets on one opposite walland an uneven number of jets on the other opposite wall. The air porthas an area, A, and a width, W, and the ratio of the area to the widthto the power of 2 (A/W²) is more than 4, preferably more than 10.

SUMMARY OF DRAWINGS

[0031] The invention will be described in more detail with reference tothe attached drawings, in which

[0032]FIG. 1 illustrates a schematic cross-sectional view of a recoveryboiler,

[0033]FIG. 2 illustrates a side view of the lower furnace of a recoveryboiler with an air port arrangement according to an embodiment of theinvention, and

[0034]FIG. 3 illustrates a plan view of the lower furnace of a recoveryboiler with an arrangement of air jets according to an embodiment of theinvention.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

[0035]FIG. 1 illustrates a conventional recovery boiler. The boiler 1comprises a furnace 2 provided with a bottom, boiler walls 4, and asuper heater 5. In the combustion process, a bed of dried and partlyburnt black liquor is formed at the bottom of the furnace. Meltchemicals flow through the porous bed to the bottom of the furnace, fromwhere they are transferred as an overflow via melt chutes to adissolving tank 7. Black liquor is introduced to the furnace throughopenings in zone 8. Air is introduced from three different levels:primary air ports 9, secondary air ports 10 and tertiary air ports 11.

[0036] As known, the recovery boiler furnace has a front wall, a rearwall and side walls. Black liquor spraying devices are disposed on thesewalls at one or several levels. A plurality of air ports are located onseveral horizontal levels on said walls for introducing air into thefurnace from an air supply.

[0037] The air ports of the furnace for supplying secondary air arearranged in a specific way. In connection with this invention, thesecondary air is used to refer to the air that is introduced between thelowest air level, i.e., the primary air level, and the black liquorspraying level or levels. In the arrangement of the invention thesecondary air is supplied as interlaced jets of air projected fromopposite walls on at least two levels, preferably on two levels.

[0038] Each secondary air level has an even number of ports for jets onone opposite wall and an uneven number of ports for jets on the otheropposite wall, as shown in FIG. 3. In this interlaced pattern, an airflow coming from an air port located on a wall having an even number ofair ports is directed in between two adjacent air ports of the oppositewall having an uneven number of air ports. The air flows coming from theopposite walls by-pass each other without actually colliding with eachother. The air ports of the different air levels are located on the samewalls, e.g., on the front and rear walls.

[0039]FIG. 2 is a schematic side view of a lower portion of one wall 12in the boiler 1, such as a rear wall that is opposite to a front wall 14(see FIG. 3). The wall 12 shows the air ports 10 for the secondary air.The air ports for the primary air are below the air ports 10, but arenot shown in FIG. 2. The wall section shown in FIG. 2 is below the blackliquor injection nozzles and above the primary air ports 9. The sideedges 13 of the wall abut with other side walls 4 in the furnace. Thesecondary air ports 10 shown in FIG. 2 may be also arranged on anopposite wall 14 of the furnace (as is shown in FIG. 3) and may also bearranged on more than two walls in the furnace. The secondary air ports10 are supplied with secondary air by an air supply 18, which providesair for combustion from atmospheric air, by circulating flue gasesrecovered from the boiler, and/or from a supply of odorous gases fromanother process in the plant.

[0040] The secondary air ports are arranged it a first row at a firsthorizontal level 15 and a second row at a second horizontal level 16.The secondary air ports 10 are aligned in elevational levels one abovethe other. The air ports of each level 15, 16 are located in rows sothat there is a transverse distance L in a horizontal direction betweenadjacent ports 10 at the same level. In addition, the secondary airports may or may not be vertically aligned between the two rows 16, 15.As shown in FIG. 2, the air ports in a first elevation 15 are offsetfrom their vertically-adjacent ports in the second elevation 16 by ahorizontal offset distance D_(x). The distance D_(x) is an offsetdistance between the longitudinal center lines of two verticallyadjacent air ports. This distance D_(x) is zero for air ports that arevertically aligned between the two rows.

[0041] In FIG. 2, D₁ is a distance between longitudinal center lines aand b, which correspond respectively to vertically-adjacent secondaryair ports 10 one above the other. Similarly, D₂ and D₃ are the distancesbetween the centerlines of other pairs of vertically adjacent secondaryair ports. D₁ is generally less than 1.5×H or less than 1.5×W dependingon which number is greater. H is the height of the tallest air port 10and W is the width of the widest air port of each pair of verticallyadjacent air ports. Preferably the transverse distance (D_(x)) is lessthan 1.0×H or less than 1.0×W, whichever is greater. Typically thetransverse distance D_(x) between two vertically adjacent air ports isin a range of 0.075 to 0.16 meters. Because of the water circulation inthe cooling tubes that form the walls 4, 12, 14 of the furnace, it maybe advantageous to have the transverse distance (D_(x)) between thevertically adjacent air ports confined to the ranges stated herein.

[0042] In addition, the two secondary air levels 15, 16 are located sothat there is a vertical distance (V) between the secondary air levels,15, 16. The vertical distance V is measured as a distance in a verticalseparation between the lateral center lines (d, e in FIG. 2) of the rows15, 16 of secondary air ports. This distance V should preferably fulfilthe following formula: V/L≦0.5, where L is the distance between twoadjacent air ports on the same row 15, 16, when measured from thelongitudinal center lines of the adjacent air ports. Typically V/L is0.05-0.5, and preferably 0.25-0.5. Typically the vertical distance, V,is 1-2 meters.

[0043] The value of the distance L between secondary air ports in thesame row depends on, for example, the number of secondary air ports inthat row on the wall of the furnace. There may be an even number ofports in a row on one wall and an odd number of ports in the same row onthe opposite wall. When there is an even number of ports in a row on onewall and an uneven number of ports in the opposite row on the oppositewall, the value of L used in the above formula may be the minimum of Lvalue in the two opposing rows.

[0044] Preferably, the shape of the secondary air ports 10 is close to ahexahedral form to minimize the area of uncooled fin areas. The airports have an area (A) and a width, W. Preferably the ratio between theport area (A) and the square of the width (W) is greater than 4, whichratio may be expressed as A/W²≧4, but this ratio may also be smallerthan 4. For instance, the ratio of A/W² can vary from 5 to 10. A featureof the invention is that each air port is closer to the air port locatedabove it than to an adjacent air port at its same level. In the extremecase the vertical distance V is close to 0, whereby two air portslocated above each other are to be replaced with one air port that isvery high and narrow. Typically, the lowest primary air port level islocated about 0.7 to 1.0 meters from the floor of the furnace (from thesmelt level). The distance between the primary level and the lowestsecondary levels 15, 16 having air jets only on two walls is about0.8-1.5 meters, in which case the lowest secondary level 15 is about1.5-2.5 m from the floor of the furnace (from the smelt level).

[0045] The air ports of the same secondary air level do not have to belocated exactly at the same elevation on the opposite walls. This meansthat the air jets on the opposite walls on the same air level are notlocated in the same horizontal plane. However, the difference betweenthe elevations of the air ports of the same level on the opposite wallsis less than 10% of the depth of the furnace.

[0046] According to a preferred embodiment the air jets of the secondaryair levels are located on the front and rear walls of the furnace, butthe arrangement of the invention can be applied to the side walls of thefurnace as well.

[0047] The number of jets on the secondary air levels is characterizedby the following numbers, depending on the spent liquor dry solidscombustion capacity of a recovery boiler capacity:

[0048] where the boiler capacity is less than 500 metric tons of drysolids per day (DS/d): 1+2 jets per secondary air level (6 jets togetherin the case of two air levels).

[0049] capacity is 500-1500 metric tons D.S./d: 1+2 or 2+3 jets perlevel.

[0050] capacity is 1500-2500 metric tons D.S./d: 2+3 or 3+4 jets perlevel.

[0051] capacity is 2500-4000 metric tons D.S./d: 2+3,3+4 or 4+5 jets perlevel.

[0052] capacity is greater than (>) 4000 metric tons D.S./d: 3+4, 4+5,5+6 or 6+7 jets per level.

[0053] Where “1+2 jets per level” means that one air port providing anair jet is located on one of the opposite walls and two ports for jetsare on the other of the opposite walls. FIG. 3 shows a 2+3 arrangementof air ports on one level providing interlaced air jets.

[0054] As shown in the single secondary air port level shown in a topview in FIG. 3, the ports 10 (and hence air jets 17) are arranged suchthat there is an interlaced pattern of air jets projecting in towardsthe center of the furnace. On a first wall, such as a rear wall 12, ofthe furnace there are three air ports arranged in one elevational levelas the secondary air ports providing the three jets shown in FIG. 3. Theopposite wall, such as the front wall 14, has two air ports 10. The airports in one level do not face directly across each other on theopposite walls. Rather, the air ports at the same elevational level,e.g., secondary air levels, but on opposite walls are offset from eachother. The offset of opposite air ports on opposite walls promotes aninterlaced pattern of air jets projecting towards the center of thefurnace. The interlaced patter of air jet can be achieved by arrangingthe ports at the same elevational level such that an odd number of portsare on one wall and an even number of ports on the opposite wall of thefurnace.

[0055] The velocity of the secondary air supplied through the air portsinto the furnace is preferably at least 40 m/s (meters per second). Inorder to prevent the formation of vertical jets that may punch the finalcombustion area where the final combustion of the unburned gases shouldtake place, the number of air jets on each tertiary air level in theclaimed arrangement is higher than the number of the air jets on thesecondary air levels. Preferably, the vertical distance between thelowest tertiary air level and the black liquor spraying level is morethan two times greater than the vertical distance between each secondaryair level.

[0056] The combustion air supply 14 can be connected to means forconveying flue gas from the recovery boiler in order to recirculate aportion of the flue gas into the furnace. The air supply 14 can also beconnected to a line for odorous gases for introducing the gases into thefurnace.

[0057] While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

[0058] The invention covers also an embodiment according to which theair jets described above form a first set of air jets. In addition tothe first air jets, a second set of air jets is arranged on the oppositewalls at the horizontal air levels so that first and second air jetsform opposed pairs and the air stream through each second jet is lessthan 25% of the air stream of the opposed first jet. The major part,i.e., more than 75%, of the air is introduced through the first jets.The air ports for the second air jets are arranged one above the otherin the same manner as the first air ports described above.

[0059] According to another embodiment of the invention a second set ofair ports is arranged close to or at the corners of the furnace, wherebythe air stream of the second jets is less than 25% of the air stream ofthe first jets. Preferably, one second air port is located at eachcorner. Gas streams in the corner areas of the furnace can be controlledby means of these weaker air jets. However, preferably the entire airstream is introduced into the furnace through the first set of jets, inwhich case the furnace has no second air jets. flue gas into thefurnace. The air supply 14 can also be connected to a line for odorousgases for introducing the gases into the furnace.

[0060] While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

[0061] The invention covers also an embodiment according to which theair jets described above form a first set of air jets. In addition tothe first air jets, a second set of air jets is arranged on the oppositewalls at the horizontal air levels so that first and second air jetsform opposed pairs and the air stream through each second jet is lessthan 25% of the air stream of the opposed first jet. The major part,i.e., more than 75%, of the air is introduced through the first jets.The air ports for the second air jets are arranged one above the otherin the same manner as the first air ports described above.

[0062] According to another embodiment of the invention a second set ofair ports is arranged close to or at the corners of the furnace, wherebythe air stream of the second jets is less than 25% of the air stream ofthe first jets. Preferably, one second air port is located at eachcorner. Gas streams in the corner areas of the furnace can be controlledby means of these weaker air jets. However, preferably the entire airstream is introduced into the furnace through the first set of jets, inwhich case the furnace has no second air jets.

1. A furnace of a recovery boiler comprising: a front wall, a rear walland side walls to the furnace, at least one black liquor spraying devicedisposed at or above a black liquor spray level on at least one of saidwalls, and a first horizontal level of air ports arranged on oppositewalls of said furnace and a second horizontal level of air ports on saidopposite walls, where said first and second horizontal levels of airports are vertically lower on said walls than the black liquor spraylevel, the air ports in each of said first and second horizontal levelson one of said opposite walls are an even number of ports and odd innumber on the other of said opposite walls, and the air ports in each ofsaid horizontal levels on said opposite walls are such that the airports for each horizontal level project an interlaced pattern of airjets into the furnace from said opposite walls. 2 A furnace inaccordance with claim 1, wherein said opposite walls are the front andrear walls of the furnace. 3 A furnace in accordance with claim 1,wherein a distance V is at least one vertical direction between lateralcenterlines of the air ports in the first and second horizontal levels,and the distance V complies with a formula: V/L≦0.5, where L is adistance between two vertical centerlines of adjacent air ports in thefirst or second horizontal levels. 4 A furnace in accordance with claim3, wherein V/L is a ratio in a range of 0.25 to 0.5. 5 A furnace inaccordance with claim 3, wherein the vertical distance V is a range of 1to 2 meters. 6 A furnace in accordance with claim 1, whereinsubstantially no air is introduced through walls of the furnace otherthan through the opposite walls.
 7. A furnace in accordance with claim1, wherein a number of air ports on each level of said first and secondhorizontal air levels is three, of which one port for each level islocated on one of the opposite walls and two ports of each level arelocated on another of the opposite walls, wherein the recovery boilerhas a capacity of no greater than 500 metric tons of dry solids per day(DS/d).
 8. A furnace in accordance with claim 1, wherein a number of airports on each level of said first and second horizontal air levels isthree, of which one port for each level is located on one of theopposite walls and two ports of each level are located on another of theopposite walls, wherein the recovery boiler has a capacity in a range of500 and 1500 metric tons DS/d.
 9. A furnace in accordance with claim 1,wherein a number of air ports on each level of said first and secondhorizontal air levels is five, of which two ports for each level arelocated on one of the opposite walls and three ports of each level arelocated on another of the opposite walls, wherein the recovery boilerhas a capacity in a range of 500 and 4000 metric tons DS/d.
 10. Afurnace in accordance with claim 1, wherein the air ports on each levelof said first and second horizontal air levels are seven ports, of whichthree ports for each level are located on one of the opposite walls andfour ports of each level are located on another of the opposite walls,wherein the recovery boiler has a capacity in a range of 1500 and 4000metric tons DS/d.
 11. A furnace in accordance with claim 1, wherein anumber of air ports on each level of said first and second horizontalair levels is nine, of which four ports for each level are located onone of the opposite walls and five ports of each level are located onanother of the opposite walls, wherein the recovery boiler has acapacity in a range of 2500 and 4000 metric tons DS/d.
 12. A furnace inaccordance with claim 1, wherein each horizontal level has at leasteleven air ports, wherein the recovery boiler has a capacity of at least4000 metric tons DS/d.
 13. A furnace in accordance with claim 1, whereinfirst horizontal level is above a level of primary air ports in thefurnace.
 14. A furnace in accordance with claim 13, wherein the level ofprimary air ports includes an upper level of the primary air ports eachhave a vertical centerline that is offset horizontally from a verticalcenterline of the ports in the first horizontal level of air ports. 15.A furnace in accordance with claim 1 wherein a velocity of air jetspassing through the air ports on said at least two air levels is atleast 40 meters per second (m/s).
 16. A furnace in accordance with claim1 wherein the furnace has at least one tertiary level of air ports isarranged above the black liquor spraying level, and the air ports onsaid at least one tertiary level are greater in number than a number ofair ports on the second horizontal level.
 17. A furnace in accordancewith claim 16, wherein a vertical distance between a lowest of the atleast one tertiary air level and the black liquor spraying level is atleast two meters.
 18. A furnace in accordance with claim 1, wherein theair ports of said first and second horizontal levels are alignedsubstantially vertically.
 19. A furnace in accordance with claim 1,wherein the air ports for the first horizontal level each form avertical centerline that is laterally offset from a vertical centerlineof an adjacent air port in the second horizontal level, and the offsetis a transverse distance D that is less than 1.5×H or less than 1.5×W,where H is a height of a tallest air port and W is a width of a widestair port.
 20. A furnace in accordance with claim 19, wherein thetransverse distance D is in a range of 0.075 meters to 0.16 meters. 21.A furnace in accordance with claim 1, wherein an air supply to the airports is in fluid communication with flue gas from the recovery boilerto recirculate a portion of the flue gas to the furnace.
 22. A furnacein accordance with claim 1, wherein an air supply is in fluidcommunication with a supply of odorous gases for introducing the odorousgases to the furnace.
 23. A furnace in a recovery boiler comprising:walls around a combustion area of the furnace, wherein said wallsinclude a first and second wall opposite each other; at least one blackliquor sprayer disposed on one or more of walls at a black liquor sprayelevation level on said walls, and a plurality of secondary air portsarranged in at least two rows on said first and second opposite walls,said ports provide jets of secondary air into the furnace, wherein saidrows of secondary air ports are above on said walls a lower level of airports and below the black liquor spray elevation level, and in each row,an even number of jets of secondary air projects from the ports in thefirst wall and an odd number of jets project from the second wall toform an interlaced pattern of air jets, and the secondary air ports havean area A and a width W and the ratio of A to W² is at least four.
 24. Afurnace in accordance with claim 23 wherein the ratio of A to W² is morethan
 10. 25. A furnace in accordance with claim 23 wherein the furnacehas at least one level of tertiary air ports arranged above the blackliquor sprayer, and wherein a number of air ports on said at least onelevel of tertiary air ports is greater than a number of secondary airports.
 26. A furnace in accordance with claim 23 wherein an air velocitythrough the secondary air ports is at least 40 m/s.
 27. A furnace inaccordance with claim 25 wherein a vertical distance between the atleast one tertiary air level and the black liquor sprayer is more than 2meters.